1. Field of the Invention
The present invention relates to a ferroelectric thin film device having a Si substrate, and more specifically, relates to a ferroelectric thin film device comprising a highly-oriented ferroelectric thin film provided on a Si substrate, and suitable for use as capacitors for DRAM and ferroelectric RAM (Fe RAM), as well as for application to a pyroelectric element, a micro-actuator, a thin film capacitor, a small piezoelectric element, and the like. The present invention also relates to a production method for the ferroelectric thin film device.
2. Description of the Related Art
In recent years, studies have actively been conducted on formation of thin films of Pb type and non-Pb type perovskite compounds such as BaTiO.sub.3 (abbreviated to "BTO" hereinafter), SrTiO.sub.3 (abbreviated to "STO" hereinafter), (Ba, Sr)TiO.sub.3 (abbreviated to "BSTO" hereinafter), PbTiO.sub.3, (Pb, La)TiO.sub.3 (abbreviated to "PLT" hereinafter), PZT, PLZT, Pb(Mg, Nb)O.sub.3 (abbreviated to "PMN" hereinafter), and the like.
Particularly, when a Pb type perovskite compound having high residual polarization, such as PZT, PLZT, or the like can be epitaxially grown, spontaneous polarization can be arranged in one direction, thereby obtaining higher polarization values and switching characteristics. Therefore, under present conditions, application to a high-density recording medium is significantly increased, and there is thus a strong demand for developing a technique for epitaxially growing a Pb type perovskite compound.
However, in application in which spontaneous polarization is arranged in one direction, e.g., in the direction of the film thickness, a structure in which a ferroelectric thin film is held between conductive layers (electrode layers) on a Si substrate, i.e., a metal-ferroelectric material-metal (MFM) structure, is required. A triaxially-oriented ferroelectric oxide thin film having good crystallinity is difficult to obtain from the following reasons:
(1) When a metal thin film of Ag, Au or the like is formed as a conductor on a Si substrate, mutual diffusion occurs between the metal thin film and the Si substrate as a base during growth of the ferroelectric oxide thin film.
(2) A method using a Pt thin film as a metal thin film can be considered. Although Pt can be epitaxially grown on an oxide single crystal substrate such as MgO, SrTiO.sub.3, or the like, direct epitaxial growth of Pt on a Si substrate cannot be realized at present.
(3) A method using an oxide such as (La, Sr)CoO.sub.3 (abbreviated to "LSCO" hereinafter) for a conductive thin film can be considered. In this case, it is necessary to insert other layers between the Si substrate and the LSCO layer, for example, as in PLZT/LSCO/BiTO/YSZ/Si, thereby causing difficulties in improving epitaxy of the ferroelectric layer as the uppermost layer. Here, BiTO represents Bi.sub.4 Ti.sub.3 O.sub.12, and YSZ represents ZrO.sub.2 to which Y (yttrium) is added.
(4) There is also a method comprising forming a TiN thin film by ion beam deposition, forming SrRuO.sub.3 (abbreviated to "SRO" hereinafter) as a buffer layer on the Pt thin film formed on the TiN thin film, and the epitaxially growing a (Ba, Sr)TiO.sub.3 (BSTO) thin film thereon. This method has problems in that a multilayer structure of BSTO/SRO/Pt/TAN/Si causes a significant increase in cost, and deterioration in crystallinity with lamination of the thin films. In addition, SRO is considered to impart ferroelectricity to BSTO by applying a stress stain to BSTO, but there is now no success of growth of a Pb type perovskite compound.
For the forgoing reasons, there is a need for a ferroelectric thin film device comprising a ferroelectric thin film of a perovskite oxide, which is highly oriented and formed in a simple film structure on a Si substrate without requiring a complicated multilayer structure, and a method of producing a ferroelectric thin film device, which is capable of efficiently producing the ferroelectric thin film device.